Question: Simplify the following expression and state the condition under which the simplification is valid. $a = \dfrac{y^2 - 36}{y - 6}$
Solution: First factor the polynomial in the numerator. The numerator is in the form ${a^2} - {b^2}$ , which is a difference of two squares so we can factor it as $({a} + {b})({a} - {b})$ $ a = y$ $ b = \sqrt{36} = -6$ So we can rewrite the expression as: $a = \dfrac{({y} {-6})({y} + {6})} {y - 6} $ We can divide the numerator and denominator by $(y - 6)$ on condition that $y \neq 6$ Therefore $a = y + 6; y \neq 6$